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MARGARET WARD ORSINI and NEENA B. SCHWARTZ

Summary.

Measurements of pituitary lh content, uterine wet and dry weights, and ovarian and pituitary wet weights were obtained for each day throughout pseudopregnancy in the hamster, and were compared with comparable data obtained during the oestrous cycle.

Ovarian weights increase during pseudopregnancy, reflecting activation of corpora lutea; they remain higher than oestrous cycle values even after ovulation recurs, reflecting delayed involution of the corpora lutea. Ovarian weight does not show a regular cycle during pseudopregnancy, in contrast with the regular changes seen during the oestrous cycle.

Uterine wet and dry weights continue to manifest a 4-day periodicity during pseudopregnancy, a finding which parallels oestrous cycle values but at a higher level.

During pseudopregnancy pituitary lh content rises, reaching a peak level on the 7th day; a significant drop occurs at the time of the ovulation at the end of pseudopregnancy. The rise exceeds that seen during the oestrous cycle and correlates with the block to ovulation in pseudopregnancy. Pituitary weights are higher than in cyclic animals, but do not show cyclic changes.

AUDREY S. BINGEL and NEENA B. SCHWARTZ

Summary.

The time at which ovulation occurred on the morning of oestrus in unmated cyclic mice was investigated. Ovulation was found to occur during the late part of the dark period and early part of the light period (lights on from 05.00 to 19.00 hours). Barbital was administered at different times before the expected time of ovulation in an effort to determine the time of the ovulatory release of lh. Barbital injection at 17.00 hours, but not at 21.00 hours, on the day of pro-oestrus prevented the appearance of ovarian evidence of lh release as indicated by the absence both of ovulation and of stimulated follicles.

AUDREY S. BINGEL and NEENA B. SCHWARTZ

Summary.

The time at which ovulation occurred in post partum mice under L14:D10 (lights on from 05.00 to 19.00 hours) was investigated. Ovulation tended to occur during the late part of the dark period and early part of the light period, but the exact time depended on the time of delivery. Mice that gave birth between 21.00 and 01.00 hours ovulated approximately 26 hr after delivery. The interval between delivery and ovulation decreased the later in the day that delivery occurred, until it became 12 to 13 hr for mice that gave birth between 17.00 and 21.00 hours.

Barbital was administered at various intervals after delivery in an effort to determine the time of release of the lh responsible for ovulation. Although the data indicate that lh release tended to occur in most mice during the afternoon or early evening, there was no one time of day at which barbital was totally effective in blocking lh release for mice giving birth during different times of the entire 24-hr light and dark period.

NEENA B. SCHWARTZ and WILLIAM L. TALLEY

In female rats, uterine weight increases early in pro-oestrus (Astwood, 1939; Schwartz, 1964), behavioural oestrus occurs during the early evening of prooestrus (Boling, Blandau, Soderwall & Young, 1941) and pituitary lh content drops from a maximum at pro-oestrus to a minimum on the morning of oestrus (Schwartz & Bartosik, 1962; Schwartz, 1964), by which time ovulation and vaginal cornification have occurred. A great deal of evidence suggests that these changes are due to a cyclic increase in ovarian secretion (see Schwartz, 1964). In a recent investigation of the timing of this ovarian secretion it was demonstrated that ovariectomy at 16.00 hours on the day before pro-oestrus, but not at 10.00 hours on the morning of pro-oestrus, blocked vaginal cornification; however, it was necessary to perform the ovariectomy at 10.00 hours on the day before

AUDREY S. BINGEL and NEENA B. SCHWARTZ

Summary.

Body, uterine, ovarian and pituitary weights, and pituitary lh content were determined on each day of the oestrous cycle of individually housed, spontaneously cyclic mice. Uterine and pituitary weights varied significantly; uterine weight was maximum at pro-oestrus and pituitary weight was maximum at oestrus. The cyclic changes in the vaginal smear cells were correlated with the cyclic ovarian changes; the morning of di-oestrus I (2 mornings after newly ovulated ova are found in the oviduct) was the most useful marker for cycle stage, since it could always be recognized as the 1st morning on which only leucocytes and nucleated epithelial cells (no cornified cells) were present in the vaginal smear. In additional studies, females, which had been previously grouped (ten/cage), were paired individually with males and killed on days of 'induced' pro-oestrus (pre-ovulatory) or oestrus (post-ovulatory). The organ weights and pituitary lh content of these mice did not differ significantly from those obtained for spontaneously cyclic mice on the corresponding 2 days of the cycle.

NEENA B. SCHWARTZ and WILLIAM L. TALLEY

Summary.

Daily measurements of pituitary lh content during pregnancy in the rat, by the ovarian ascorbic acid depletion method, reveal no evidence of the regular changes manifested during the non-pregnant oestrous cycle. Instead, lh content rises from typical oestrous values on Day 1 to levels characteristic of cyclic pro-oestrus on Day 8. Pituitary lh content then exceeds pro-oestrous levels throughout the remainder of pregnancy until Day 21. Ovarian weight first decreases up to Day 10, then rises until the end of pregnancy. Ovarian histology reveals the presence of follicles of all sizes on each day, accompanied by increasing size of the corpora lutea of pregnancy. The significance of these observations and previous data in the literature concerning the pituitaryovarian axis during pregnancy are discussed from the point of view of the question of persistence of some manifestations of the oestrous cycle during pregnancy.